By John Fauber and Susanne Rust,Milwaukee Journal Sentinel

MILWAUKEE – Scientists don’t know when the next super flu virus will hit the United States, but they are certain about one thing: It will arrive on wings.

There are two especially worrisome scenarios in which an avian flu capable of producing a pandemic travels to North America.

In one, it is brought here by migratory birds that travel between Northeast Asia and Alaska. In the other, the virus already has made the genetic changes that allow it to be readily transmitted from person to person, and it arrives on a commercial jet.

In the latter scenario, it will have begun spreading among people in Asia. Someone who has been infected but is still a day or two away from showing symptoms will get on a plane headed for the United States. Others on the plane will become infected, and they, in turn, will take their illness to destinations around the country.

Eventually, nearly 30 percent of Americans could get sick.

“That would be the quickest, scariest and easiest way,” said Anne Moscona, an infectious disease expert at Weill Cornell Medical Center in New York.

A flu strain with a long incubation period, such as the bird flu, or H5N1, would facilitate such a transcontinental spread, said Moscona, a professor of pediatrics and of microbiology and immunology.

Bird viruses can infect people when the genetic material in them – RNA – undergoes changes that make the transmission from person to person easier.

There are two ways for this to happen. In the first, the bird virus itself can transform or mutate to adapt to people – this is called antigenic drift. That’s what is believed to have happened in the deadly 1918 pandemic. A bird flu mutated in such a way that it could break into human cells. The consequences were devastating.

“There is increasing evidence that H5N1 is trying to do that,” said Kelly Henrickson, an associate professor of pediatrics and infectious disease expert at the Medical College of Wisconsin. “It’s becoming more efficient.”

The other way a flu virus can jump from bird to person is via an intermediary, such as a pig.

Pigs have receptors on their cells that recognize and accept both human and bird flus. And people have cells that recognize and accept swine flus. In a process called antigenic shift – as opposed to antigenic drift – a virus from a bird and person each get into the same pig cell, and the two viruses exchange segments of their RNA to create an entirely new, dangerous virus.

It works kind of like the way a soft-serve ice cream machine does. Imagine the pig as the machine. Chocolate ice cream goes in one side, vanilla in the other. The two flavors mix, and out comes a new variety with both the original flavors.

The machine could also be a person. In this case, a person is carrying a human flu – like the one we are being vaccinated for this year – and is exposed to avian flu. The two viruses mingle, and out pops a new virus with the virulence of the bird flu and transmissibility of the human flu.

In both scenarios, there couldn’t be a better place on Earth than Asia – particularly eastern China, Thailand, Vietnam and Indonesia – for this kind of mixing to occur.

“The genetic mixing pot of China is mixing up new viruses all the time,” said G. Richard Olds, a professor of medicine and infectious disease expert at the Medical College of Wisconsin and Froedtert Memorial Lutheran Hospital.

This part of Asia is a nexus point for the intermingling of wild and domestic fauna. Located on migratory bird routes, the region allows for frequent contact between wild birds, domestic poultry, pigs and people.

Wild birds are the natural hosts of flu. They carry around every combination of flu virus: H5N1, H1N1, H2N3, etc. The disease rarely makes them ill. But when it does, it causes serious concern among avian pathologists.

In many parts of Asia, pigs and ducks follow their owners around much as free-range farm dogs do in the United States They go to work with people – spending the days in rice paddies and fields – and return home with them at the end of the day. This lifestyles allows viruses to jump into a variety of hosts, such as birds, pigs and people, in which they can mingle, adapt and transform.

Carol Meteyer, an avian pathologist with the U.S. Geological Survey’s National Wildlife Health Center in Madison, said her antenna went up when she first heard reports from China about open-billed storks dying of flu in late 2003 and early 2004.

“I thought, oh my gosh, this influenza is doing something different in storks” than the typical avian influenza, she said, adding that avian flus generally do not kill their wild, migrating hosts.

Outbreaks continued in chickens in Asia, and this year there were reports of bar-headed geese in eastern China dying from the influenza that had been killing poultry.

“That was pretty much the clincher that the virus had changed its M.O.,” Meteyer said.

What researchers think happened is that wild birds – the natural reservoir of avian flu – shed their benign virus, probably through feces, where domestic ducks or other poultry picked it up. Once in these new birds, the virus mutated or mixed – making the domestic birds sick. Then these birds shed the new, transformed virus, which could now kill migratory birds.

“We don’t know how freely the virus is moving between domestic and wild birds,” Meteyer said.

However, the only way to get rid of the disease is to cull all domestic poultry anywhere the disease is found, she said. Then, the infected area also should remain bird-free for at least a couple of months.

“You need to break that chain,” she said, and cut the disease cycle.

There’s also the possibility the disease could arrive in North America via birds – before it mutates into a human pathogen in Asia.

Concerned about this scenario, researchers have started looking in Alaska, and elsewhere, for the virus. According to Kevin Winker, an associate professor and curator of birds at the University of Alaska Museum in Fairbanks, migratory birds move freely between Asia and North America.

“Western Alaska is the link between Asian birds and North American birds,” he said. “Given time, H5N1 or its component genes is likely to cross from Asia to Alaska.”

That means wild migrating birds in North America eventually could infect domestic poultry – disrupting the economy and possibly providing another route for human infections on this continent.

Ducks and geese that pass through China live in the waters of Alaska where, in turn, they mingle with North American birds that travel south along the West Coast and east through Canada.

At the same time, North American birds such as sandhill cranes travel to Siberia, where they breed and mingle among Asian birds.

But while migratory routes provide a means for H5N1 to arrive in America, Winker said he is even more concerned about the virus getting here through commercial poultry trade, a concern that has prompted the U.S. Department of Agriculture to monitor poultry markets.

While the arrival of the virus in the United States in birds eventually could pose a hazard to humans, officials said its more immediate impact could be to cause chaos and huge financial losses for poultry producers.

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